In general, mass spectrometry allows the identification and quantification of atoms and molecules (hereinafter: “molecules”) that can be ionized. Mass spectrometry is commonly used to characterize the molecular composition of a surface. It is a powerful method to detect, identify, and quantify molecules of different masses. Additional spectra obtained after fragmentation of a molecule can be performed for unambiguous identification of most molecules.
For certain applications like the characterization of surfaces in material sciences or the diagnostics of diseases like cancer, an image with a spatial resolution of the mass spectrum is required.
There are several different methods known to generate a maximum number of ions during each measurement cycle, a prerequisite for fast image acquisition:
For matrix assisted LASER desorption ionization (MALDI) (see: A. F. Maarten Altenaar, PhD-thesis, University Utrecht, Netherlands, 2007), a LASER beam is focused on one spot of the surface analyzed. In most cases, a matrix consisting of an organic compound with high light absorption embeds the molecules to be investigated. A focused laser beam ionizes a small portion of the surface area and a large number of ions are generated by a charge transfer from the ionized matrix to the bio-molecules. The exact mass for each of the resulting ionized molecules is determined by mass spectrometry in a subsequent step, typically in a time of flight mass analyzer. The lateral resolution achieved today comprises about 5 to 10 μm reflecting the maximal focussing of the LASER beam. In rare cases, a resolution of 1 μm can be achieved.
Other methods include fast atom bombardment (FAB). FAB destructively induces the ionization at the point to which the ion beam is focused on the surface. The ion gun generates a fast ion beam that consists for example of In+ or Ga+ ions. As soon as the accelerated ions impact on the target surface, a multitude of small, ionized fragments are generated from the bio-molecules present at this spot. Although FAB massively decomposes the surface of the sample, the ion beam can be focussed to a spot of less than 1 μm in diameter. One of the disadvantages of this technique relates to the strong impact of the ions on the surface which causes a substantial fragmentation of molecules, especially bio-molecules. A variation of FAB employs a liquid metal ion source. For example Bismuth ions can be used as ionization source.
Alternatively, the surface can be bombarded with Fullerenes (C60) that dissipate the kinetic energy upon impact on the surface and therefore lead to a softer ionization. Thus, less fragmentation of the ions generated is caused and therefore larger molecules may be ionized. This technique can be utilized to generate ions from a sub-micrometer spot on a sample.
Subsequently the ionized molecules are separated according to their specific m/z value. Here, m designates the mass and z designates the electric charge of the ion. The standard mass analyzers use either the time of flight (TOF) or quadrupole (Q) mass selection principle. Alternatively, ions may be trapped in an ion trap and their mass determined when expelled from the ion trap during a frequency scan.
Also, ions may be introduced into a cyclotron and their mass is determined based on their resonance frequency in a frequency scan. The m/z value of a molecule is correlated to the resonance frequency and determined by a Fourier transformation of the frequency spectra measured. This non-destructive principle of mass determination, also referred to as Fourier Transformation-Ion Cyclotron Resonance (FT-ICR), provides very high mass accuracy.
The mass selection principles outlined above, or any homo- or hetero-mer combination of these methods, are conventionally known. For an un-ambiguous identification of a molecule, secondary ions are generated in a collision cell located between two mass selecting units. For example, in a TOF/TOF setup secondary ions are generated in a collision cell.
An already well-known technology to reconstruct mass spectrometric images of a two dimensional surface is based on a single spot analysis (scanning principle). The ionization is induced in a small region (hereinafter: “spot”) on the sample surface. A typical spot size is typically around 10 μm to 100 μm in diameter. The ions generated in the spot are detected in a mass spectrometer, e.g. a standard time of flight mass spectrometer. This allows determining not only the exact mass but also the abundance of each specific ion within the spot.
The mass spectrometer acquires mass spectra of adjacent spots and thereby scans the surface. Following data acquisition, a four-dimensional map or “picture” is assembled in which the x- and y-axis reflect the imaged surface and the z-axis the mass spectrum. The fourth dimension represents the ion rate measured. An image reflecting the ion abundance distribution of one m/z value over the surface can then be easily visualized.
When mass spectrometric images are reconstructed based on the scanning principle, the mass spectrometer scans the surface along a predetermined path and with a defined step-width. Typically, the instrument acquires more than a hundred and up to one thousand spectra at each individual spot. First, the mass spectrum from each point on a surface is recorded separately. Based on this information, the mass spectrometric image is reconstructed from the acquired mass spectra of each individual spot by a point by point reconstruction. Because a high number of repeated measurements at one spot are necessary and the mass spectra are acquired in each spot individually, the process of data acquisition is very time-consuming. In a variation of it, the long data acquisition time is reduced. In this case, the mass spectrum is only acquired at pre-determined spots on the surface and later an extrapolation allows to determine a subset of area to which this particular spectrum might fit additionally.
In FIG. 11, a flow chart of a duty cycle according to the conventional method is shown. First, in step S200, the system is initialized. In step S201, a start location (X,Y)n with n=1 on the sample is determined, where the first measurement cycle is performed on. n designates the number of locations to be scanned. The ionization beam, e.g. the laser beam or an atom beam, is focused to the predetermined location, S202.
A start time of the measurement is set in S203, e.g. by detecting the laser pulse used for ionization. A spot of the sample at the predetermined location (X,Y)n is ionized by the ionization beam pulse in step S204. In step S205, the generated ions drift towards the detector and may generate an amplified signal that impinges the detector. In the detector, the number of events per Time of Arrival ToAn (X,Y)n is counted (step S206), where the Time of Arrival is measured relative to the start time. If enough measurements have not been performed for a predetermined location yet to obtain a sufficient statistics, it is decided in S207, to return to S203. Otherwise, it is checked in step 208, if all locations of interest of the sample were investigated. If this is not the case, a next location for an investigation is determined, S209, and the procedure returns to step S202.
If all locations have been scanned (“yes” in step S208), the measurement part of a duty cycle is finished and a 4-dimensional image may be reconstructed from all cycles of all locations (S210). For that, the data may be arranged in sets of substantially the same Time of Arrival [ToA], representing a certain m/z ratio, location (X,Y)n, and the number of events. If the number of measurement cycles was different at different locations, the number of events at a location should be normalized to the number of cycles at this location. Finally, in step S211 the data may be evaluated and presented. The duty cycle is finished in S212.
In order to reconstruct an image of an extended surface with one of the above schematically described instruments, a scanning process is required. Moreover, determined through the principle of measurement (maximal number of ions per duty cycle), the duty cycles for each measurement are relatively long. Typically, the duty cycle for the determination of a full mass spectrum image requires the number of ionization shots needed to acquire a spectrum with a given number of detected ions multiplied by the number of spots investigated to reconstruct the mass spectrum image.
The mass spectrometers of e.g. the TRIFT series are another attempt to provide a spatially resolved mass spectroscopic image of a surface (A. F. Maarten Altenaar, PhD-thesis, University Utrecht, Netherlands, 2007). The mass spectrometer of the TRIFT series is described here as an example for a mass spectrometer which is used in quality control during semiconductor microcircuit production, or in the investigation of surfaces of biological samples as described in the publication mentioned above.
In such a spectrometer, a time of flight mass separator is used to acquire the two-dimensional image for a very narrow range of m/z values. The time of flight mass separator is constructed such that it provides directional and velocity focussing properties (double focussing), that enable the arrival of a focussed ion image for a selected m/z value at the detector. In this instrument setup three electrostatic field sectors (for example Matzuda plates) are arranged at a 90° angle to each other along the flight path of the ions providing double velocity focussing.
The TRIFT series of time of flight mass spectrometers can image a surface. It can be operated either in a stigmatic or in an astigmatic mode. In the stigmatic mode, the spatial relationship of the ions is preserved until arrival at the detector. A large amount of molecules are ionized on a restricted surface area either with an ion beam or a laser beam illuminating the surface. The ionized molecules are accelerated. One m/z target value (or a narrow m/z target range) is selected during the drift phase by two blankers included in the instrumentation that enable the selection of a narrow m/z window that is observed at the detector. The position sensitive detector records the spatial distribution of the ions within the selected mass range. In detail, after signal conversion and amplification through a microchannel plate, a phosphor imaging screen converts electrons to light that is detected by a CCD camera.
Whereas this instrument images a surface area with spatial resolution, it is limited in its capacity and speed by at least three factors:
First, the instrument is only capable of generating three-dimensional data and not four-dimensional datasets. For each measurement cycle, a narrow m/z range of interest has to be selected, for which a mass spectrometric image is acquired. Therefore, all ions of different m/z values than the selected window are lost during each duty cycle. Typically, the duty cycle for the determination of a full mass spectrum image requires the number of ionization shots needed to acquire a spectrum with a given number of detected ions multiplied by the number of m/z ranges investigated to reconstruct the mass spectrum image.
Second, the ion signal is converted in an electron signal which induces a light signal at a phosphor-screen which is finally detected by a CCD camera. Although this is a standard procedure for acquiring images of signals converted to a light pulse, it restricts the efficiency of image acquisition by this three step conversion.
Third, the image acquisition requires several milliseconds due to the method used to record the light signal. The read out process of the CCD camera takes much more time than the duty cycle of the whole instrument. The long duty cycle is due to the photo sensor array used in CCD cameras which finally restricts the rate of image acquisition because sample ionization and mass separation within a time of flight mass spectrometer is achieved within microseconds.
In another attempt to imaging mass spectrometry a three layer delay-line anode was used (O. Jagutzki, V. Mergel, K. Ullmann-Pfleger, L. Spielberger, U. Spillmann, R. Dörner, H. Schmidt-Böcking: A broad-application microchannel-plate detector system for advanced particle or photon detection tasks: large area imaging, precise multi-hit timing information and high detection rate, Nucl. Instr. and Meth. in Phys. Res. A, 477 (2002) 244-249). It comprises three individual delay chains. From the relative delay of the signals arriving at the two ends of each delay chain, the position of a single event on that delay line can be obtained. With two independent delay lines, the determination of the location in a detection plane is not unambiguous, if two events occur at the same time at different locations. A third delay line allows for unambiguous identification even in this case of two simultaneous events but still the detector may suffer from ambiguities in the determination of an impact position at higher intensities.